Heretofore, there has been known an electric motor including a rotor and a bearing rotatably supporting the rotor, wherein an axial load is preliminarily applied to the bearing as a preload (as used in this specification, the term “electric motor” has a concept including a generator and a generator-motor which are based on the same principle as that of an electric motor). The applied preload eliminates gaps between elements which constitute the bearing, thereby enhancing the rigidity of the entire bearing. Each of JP 07-248023A (Patent Literature 1) and JP 2006-97765 (Patent Literature 2) discloses a technique of applying the preload to a bearing by use of an annular spring. The annular spring is a ring-shaped leaf spring fabricated to have a circumferentially wave.
Either of the techniques disclosed in the Patent Literatures 1 and 2 is, however, incapable of positively restraining the bearing from a vertical movement, i.e., wobbling in a thrust direction. Particularly, in a vertical electric motor, more particularly, in a vertical electric motor to be installed on a vehicle such as a hydraulic excavator, it is likely to occur that a vertical load, i.e., a thrust load, due to vibrations or the like acts on the bearing to bring the bearing into vertical movement, which generates repeated large flexural deformation in the annular spring to thereby accelerate degradation of the annular spring. Moreover, the flexural deformation and degradation of the annular spring can make the magnitude of the preload deviate from an adequate range to exert a negative influence on the bearing itself.